Increasingly miniaturized system-in-package (SiP) devices used in a growing number of static and mobile devices present problems of electromagnetic interference (EMI) to signal integrity while at the same time some of the components can radiate electromagnetic energy which can interfere with other circuit elements. For example, a SiP device packaging a digital processor chip generates electromagnetic energy which can interfere with the operation of a high sensitivity low noise radio frequency amplifier in the same SiP. The nature of these high frequency radiative signals along with the increasing proximity of components on the circuit module substrate and printed wiring board (PWB) create the need for effective isolation between components and for improved EMI shielding.
The conventional method used for EMI shielding is to form a 6 sided structure, ensuring complete enclosure of the components and creating a Faraday Cage. In all cases involving circuit modules and circuit boards, a ground or reference plane in the circuit module substrate or PWB forms one side of the enclosure, while the rest of the structure may be formed using various techniques. For example, in one approach a housing assembly is used as a cover of the circuit module substrate or PWB assembly thereby providing EMI enclosure. Various cavities on the inside of the housing assembly are metallized on the interior surface and are electrically coupled to the circuit module substrate or PWB using a conductive elastomeric gasket, while the necessary contact force at the gasket interface is achieved through mechanical fasteners such as clips or screws. Another, more common approach uses preformed conductive shield cans: sheet metal enclosures with and without removable lids that attach directly to the circuit module substrate or PWB, so that the assembled SiP or PWB incorporates the shielding.
Components on the vast majority of SiPs and PWBs today and all high performance systems are assembled using surface-mount technology (SMT). This process involves screen printing solder paste onto the bare board or substrate, placement of components aligned to the printed solder paste pattern, and running the populated boards through a solder reflow oven.
When metal cans are used as shields, they will typically have an array of holes or apertures stamped into the top which permit more even heat distribution during the reflow process. The shield cans are normally stamped from sheet metal and are formed into individual rectangular boxes. They are mechanically and electrically attached to the circuit module substrate or PWB in the same soldering process as the components using ground or reference tracks provided within the component layer circuitry for this purpose, and are typically placed over the components as the last step before the board goes through the reflow oven.
Since the shield cans are covering the components during the solder reflow process, this may interfere with the thermal profile necessary to achieve well-formed and reliable solder joints. With the shield can soldered in place, inspection of solder joints becomes impossible, and rework becomes more complex and problematic should that be required. As a result, shield cans are available with snap-on lids, but this often adds size and complexity to the shield enclosure and may compromise the shielding effectiveness of the can.
In compact, high density designs, and in particular modular assemblies operating at RF equivalent frequencies packaged as components for assembly onto a motherboard, preformed metal cans become less attractive. Components are placed closer together, so the ground or reference tracks defining the perimeter of the shielded area become very narrow. This makes the use of multiple, single cavity shield cans impractical when several cavities are needed.
Another limitation of this approach derives from the assembly process. Since the shield cans are mounted and fixed by reflow of solder paste, it becomes impractical to mount them on both sides of a circuit module substrate or a circuit board, as the shield cans mounted in the first pass would fall off in the second pass unless glued in place. Gluing the cans in the first pass would eliminate any possibility for rework or inspection unless removable lid types were used, adding complexity, cost, and degrading the shielding capabilities of the enclosure.
Yet another limitation is found in the use of shield cans in that cleaning of flux residues and other byproducts of the reflow operation becomes problematic if not impossible under the shield can. Yet at the same time the shield can cannot provide a hermetic enclosure due to the need for either holes or a removable lid during the reflow process as discussed above.
Some of these issues are addressed when a Faraday Cage electromagnetic shielding enclosure may be created inside the circuit module substrate or circuit board using combinations of inner layer patterning for the top and bottom and conductive pillars or vias for the walls of the cage, however this approach requires that all components requiring shielding be placed within the cage, i.e. embedded within the circuit module substrate or circuit board. As a consequence, this approach is often impossible, not feasible, and/or too expensive to use in the construction of a high density device.